TCP that supports reliable data communication between terminal systems has been improved to be adapted to a conventional wired network. However, an effective data communication method using the TCP in a wireless network is needed to meet ever-increasing demands for Internet access in the wireless network. However, many problems may occur when a conventional TCP is directly adapted to the wireless network, because the conventional TCP interprets a problem of data packet loss as being caused by a burst of data communication demand, despite it is mainly caused by a high transmission error rate and a handoff resulting from a mobility of a wireless host.
There has been suggested a number of methods to prevent a deterioration in efficiency of TCP in the wireless network. The suggested methods are largely classified into two types: One is to modify the conventional TCP used by a wireless mobile terminal, and the other is to install a proxy in the middle of a communication path between two hosts connected through TCP. A proxy, as a solution widely used to overcome differences between network protocols, is used for a local recovery or a disconnection recovery process in data communication using wireless TCP.
In the meanwhile, mobile IP (Internet Protocol), which operates as a lower layer protocol of the wireless TCP, is a standard protocol suggested by IETF (Internet Engineering Task Force). The mobile IP includes a routing function for a mobile host in addition to-basic functions of conventional IP. A setup process of a data communication path in the mobile IP includes an advertisement, a registration and a tunneling process. Whenever a mobile host moves from a network where the mobile host currently resides (home network) to a foreign network, a foreign agent (FA) in the foreign network detects the mobility of the mobile host through the advertisement process. The foreign agent, which senses the mobility, sets up a temporary data path to an agent of the home network, i.e., a home agent (HA), through the registration and tunneling processes. Here, in the home agent, an IP address is firstly assigned to the mobile host. Thus, all data packets from hosts that are connected with the mobile host are transmitted to the mobile host via the home agent and the foreign agent.
A wireless TCP proxy, which exists in a routing path between a transmitter and a receiver, is preferably loaded on the home agent or the foreign agent for the efficiency of data transmission/reception, which means that at least two wireless TCP proxies are required in the routing path. Moreover, in order to reduce the packet loss resulting from a handoff in the mobile IP, there are suggested methods to configure the foreign agents in a tree structure. In this case, more than or equal to two TCP proxies exist in the routing path since data packets are transmitted between a transmitter and a receiver via agents as much as a depth of the tree structure.
When the foreign agents are configured in the tree structure (in case a plurality of wireless TCP proxies existing in the routing path are activated), there may be a serious deterioration in efficiency due to a malfunction in a TCP controlling algorism, which results from redundant acknowledgement (ACK) and data transmission.
Although the foreign agents are formed as not the tree structure but a single foreign agent, at least two agents, i.e., a home agent and a foreign agent exist in the data path of the mobile IP. Therefore, at least two wireless TCP proxies are always activated in data communication through the mobile IP.
The simplest method to solve the problem, which arises when a plurality of wireless TCP proxies are used in the communication path, is to fix the position of a wireless TCP proxy at the home agent. This method has an advantage that there is no overhead and packet loss due to the change of the proxy since there is no need to change the proxy even if a mobile host corresponding to the proxy moves from a home network of the mobile host to a foreign network. On the other hand, it has a disadvantage that the mobile IP, as a lower layer protocol, should support a bi-directional tunneling function. Accordingly, the method for locating the proxy at the home agent cannot be used when the bi-directional tunneling function, which is one of the option functions of the mobile IP, is not supported, or when a route optimization function supported by the mobile IP makes data packets bypass the home agent.
Another method to solve the problem is to locate the wireless TCP proxy at an agent that is nearest to the mobile host. Most of wireless TCP's based on a proxy, which have been suggested so far, use such a method. This method has an advantage that an additional bi-directional tunneling function is not required. However, it has a drawback that the wireless TCP proxy should be changed into another as the mobile host moves over boundaries between networks. Consequently, when a mobility of the mobile host is high, there is a problem that the processing overhead caused by frequent changes between proxies is increased.
Still another method is to locate the wireless TCP proxy in a certain level of a structure that is formed hierarchically. This structure is a three-level structure comprising: a mobile host; a mobile support station existing as a cell unit; and a supervisor host for managing a plurality of mobile support stations. An advantage of this method is that it can reduce a proxy change frequency according to the movement of the mobile host by locating the wireless TCP proxy at the supervisor host, while a disadvantage thereof is that there is no flexibility in coping with a varying mobility of the mobile host since the position of the proxy is fixed at the supervisor host.